Inks are often of one of two types, depending on the type of colorant used. Dye-based inks often comprise a dye dissolved in a liquid medium. Pigment inks often comprise a pigment dispersed in a particulate form in a liquid vehicle. Pigment inks tend to have better ozone-fastness and light-fastness than dye-based inks. However, because the pigment is in the form of fine particles there is a tendency for the particles to agglomerate or flocculate whilst the ink is being stored and/or whilst the ink is being used (e.g. printed). Such agglomeration or flocculation before the ink has been printed onto a substrate is highly undesirable, particularly in ink jet printing inks where the printer nozzles are very small and are susceptible to blockage by any oversized particulate matter. Thus, in the ink jet field a great deal of effort has been spent attempting to provide pigment dispersions with sub micron sized particles and to increase the colloidal stability of these pigment dispersions. It is desirable that the ink is capable of printing for long periods and to a high print quality, this is often referred to as operability.
It is also desirable to provide pigment inks which offer high optical density (OD), especially when printed onto plain paper.
Pigment dispersions are often colloidally stabilised by means of a dispersant.
In our own studies on dispersant stabilised pigment inks we have found that it is particularly difficult to prepare inks which simultaneously exhibit good colloidal stability and high OD on plain paper. For example, we have found that dispersant stabilised pigment inks known in the art having a high colloidal stability provide a low OD when printed on to plain paper and vice versa.
We have also found that the few dispersant stabilised pigment inks which do provide inks exhibiting high OD when printed onto plain paper tend to also use dispersants which require significant and undesirably high amounts of organic liquids to assist in dissolving/dispersing the dispersant, for example in the pigment dispersion or comminution step.
Further, it is desirable that a dispersant is effective in the dispersion or comminution process. Quicker comminution (e.g. milling) to submicron particles sizes saves substantial energy and it may also result in less pigment particles having a particles size markedly smaller than the target size. Particles much below the target size are often referred to as fines.
Pigment based inks when printed onto a substrate often produce final images wherein the pigment is not firmly secured to the substrate surface. Prints from pigment based inks often exhibit poor rub fastness, water-fastness or highlighter smear fastness.
Aqueous inks and ink jet printing inks require very different polymers as dispersant from organic inks. The present invention also aims to provide polymers suitable as dispersants for particulate solids in aqueous liquid vehicles. In addition the present invention attempts to provide polymers which can colloidally stabilise dispersions of particulate solids even when large amounts of organic liquid are present in combination with water.
Commercially, there still remains a need for dispersions which can be used to prepare pigment inks which solve, at least in part, one or more of the abovementioned problems.